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PT/INR Home Testing

CoaguChek XS Accuracy

The CoaguChek XS system is extremely accurate and demonstrates (97%) correlation with traditional laboratory testing methods so that you can confidently replace your lab visits with an INR test from home using the a PT/INR Meter.

For testing the meter precision and accuracy prothrombin time test results were obtained from CoaguChek XS monitors from three external sites, using both capillary and venous whole blood.

The PT results are presented in INR (International Normalized Ratio) units. The INR was created to
allow for differences in the sensitivity of different thromboplastins. The sensitivity of a thromboplastin is defined by its International Sensitivity Index (ISI). Thromboplastins with lower ISI values are said to be more sensitive to factor deficiencies. Generally, a manufacturer of a commercial thromboplastin reagent assigns an ISI to the reagent. The INR is calculated according to the following formula:

INR = (patient PT/mean normal PT)ISI

The patient PT is the patient result in seconds, the mean normal PT is the normal PT as defined by
the manufacturer, and the ISI is the International Sensitivity Index assigned to the reagent.2


The precision of a system is its ability to replicate measurements to give similar results. Whole blood precision was analyzed separately for normal donors (those patients not on warfarin) and warfarinized donors. The study measured precision for both capillary and venous whole blood on the CoaguChek XS system.

For the capillary testing, the investigator performed a duplicate fingerstick; the first drop was applied to one CoaguChek XS monitor and the second drop was applied to a second CoaguChek XS monitor. A rotation scheme was used to ensure that the same instrument was not always dosed first. For the venous testing, blood was applied from a syringe; no rotation scheme was used.


The accuracy of a system is evaluated by performing a method comparison. The samples are analyzed on the method under evaluation and the results are compared to the results from a standard laboratory reference.

In this study, the portable monitor samples were obtained by fingerstick and venipuncture and were analyzed on a CoaguChek XS System using a human recombinant thromboplastin calibrated to an ISI of 1.0. The lab reference sample was obtained using a 3.2% sodium citrate Vacutainer® and was run on a Sysmex analyzer using Dade Innovin® thromboplastin. To cover as much of the reading range as possible, the study included 100 patients per site who were being treated with warfarin therapy and 20 patients per site who were not.


Prothrombin Time testing is not standardized across systems – whether they are lab instruments or portable point-of-care monitors–and there is no defined “gold” standard for acceptable performance. However, in this comparison study, regression statistics yielded correlation coefficients of 0.97 or greater for all comparisons.
The Coefficients of Variation presented for precision of whole blood duplicates indicate that the precision of the CoaguChek XS monitor for both capillary and venous whole blood samples is very good.

These results and the findings of several independent studies support the fact that point-of-care
PT/INR testing with the CoaguChek XS monitor is a viable alternative to drawing a venous sample
and sending it the lab, in terms of both accuracy of results and the cost-effectiveness of care.3-8

1 Körtke H, Minami K, Breymann T, et al. INR self-management after mechanical heart valve replacement: ESCAT. Z Kardiol. 2001;90(6)118-124.
2 Medicare National Coverage Determinations Manual. Chapter 1, Part 3 (Sections 170-190.34) Revision 131, 02/23/2011. Available at:
downloads/ncd103c1_Part3.pdf. Accessed December 1, 2011.
3 Anderson D, Harrison L and Hirsh J. Evaluation of a Portable Prothrombin Time Monitor for Home Use by Patients Who Require Long-term Oral Anticoagulant Therapy. Arch
Intern Med. 1993;153:1441-1447.
4 More CoaguChek test strips are sold for point-of-care anticoagulation testing than all other brands combined. GHX Market Intelligence. Data on file at Roche Diagnostics.
5 CoaguChek XS System for PST [package insert 04899954001(05)]. Indianapolis, In: Roche Diagnostics; 2009.
6 CoaguChek XS PT Test [package insert 04625706001(04)]. Indianapolis, In: Roche Diagnostics; 2009.
7 Adapted from: Prothrombin (PT); Medicare National Coverage Determinations Manual; Chapter 1, Part 3, Section 190.17. Available at:
downloads/ncd103c1_Part3.pdf. Accessed December 1, 2011.

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